1. Field of the Invention
The present invention relates to a control device for an internal combustion engine which is provided with a variable valve drive mechanism, and more specifically, to an estimation device for a cylinder intake air amount and an internal EGR rate in an internal combustion engine, which serves for calculating an amount of intake air in a cylinder and an internal EGR rate with a high degree of accuracy.
2. Description of the Related Art
In general, in order to control an engine in a suitable manner, it is important to calculate an amount of air to be sucked into a cylinder with a high degree of accuracy, and to carry out fuel control and ignition control according to the amount of air which has been sucked into the cylinder.
As for the fuel control, fuel need only be controlled so that an amount of fuel is injected to provide a target air fuel ratio with respect to the amount of intake air sucked into the cylinder, but as for the ignition control, there is a need to control ignition timing to an ignition advance angle (MBT: Minimum Spark Advance for Best Torque) so as to provide a maximum output power of the engine according to not only the engine rotational speed and the amount of intake air sucked into the cylinder, but also other factors (e.g., the temperature of the engine, the situation of the occurrence of a knock, the property of fuel, the amount of EGR).
Among the above-mentioned factors which have an influence on the MBT, for example, the temperature of the engine can be detected by an engine cooling water temperature sensor, and the situation of the occurrence of a knock can be detected by a knock sensor, and the property of fuel can be determined as regular gasoline or high-octane gasoline according to the situation of the occurrence of a knock.
However, as for the amount of EGR, there has been known, as a first conventional device, a technique in which an EGR valve is arranged in an EGR passage which connects an exhaust pipe and an intake pipe with each other, so that the amount of EGR (external EGR) is controlled by the valve opening degree of the EGR valve.
In addition, as a second conventional device, there has been known a technique in which a variable valve timing mechanism (hereinafter referred to also as “VVT”) is provided which serves to make variable the valve opening and closing timing of an intake valve and an exhaust valve, so that the amount of EGR which is the exhaust gas remaining in the cylinder (internal EGR) is controlled by changing an overlap period in which the intake valve and the exhaust valve are in their open states at the same time according to the valve opening and closing timing. Moreover, the above-mentioned first and second techniques may be used at the same time.
As for the external EGR, a rough amount of EGR can be calculated from a degree of opening of the EGR valve, an atmospheric pressure, and an internal pressure in the intake pipe, but as for the internal EGR, a variety of kinds of techniques have been proposed, as will be described later. When the amount of external EGR and the amount of internal EGR can be estimated with a high degree of accuracy, it becomes able to control the engine at optimum fuel economy by correcting ignition timing in combination with other factors which have influences on the MBT.
In the past, as an estimation device for the amount of internal EGR, there has been proposed a technique in which a basic index of an internal EGR state is obtained from an overlap amount of intake and exhaust valves and an engine rotational speed, and a valve overlap center phase at which the amount of internal EGR becomes a minimum is obtained as a minimum EGR center phase, so that the basic index is corrected according to a deviation between an actual center phase of the valve overlap and the minimum EGR center phase, thereby estimating the internal EGR state (for example, refer to a first patent document).
In addition, as another estimation device, there has also been proposed a technique in which an overlap period of an intake valve and an exhaust valve is divided into a first half and a second half, and an approximate characteristic line for an amount of exhaust gas passing through the exhaust valve in each of the first and second half periods is calculated, and further, an amount of blow out gas and an amount of blow back gas are calculated by geometric calculation on the basis of each approximate characteristic line, thereby calculating an amount of residual gas (for example, refer to a second patent document).
Moreover, as another estimation device, there has also been proposed a technique in which a rate of residual gas is calculated based on an amount of residual gas at the time when an intake valve is open, and an amount of residual gas which is blown back in a valve overlap period (for example, refer to a third patent document).
In addition, in the technique described in the above-mentioned first patent document, it can also be said that a cause to increase an amount of arithmetic operation or calculation load is that there is a need to separately calculate an amount of intake air sucked into a cylinder, and hence, there has been the following problem. That is, many corrections are required in order to calculate the amount of internal EGR with a high degree of accuracy, and accordingly, the number of maps to be required becomes huge, and besides, it is also necessary to separately calculate the amount of intake air sucked into the cylinder.
Moreover, in the conventional technique described in the above-mentioned second patent document, the overlap period of the intake valve and the exhaust valve is divided into the first half and the second half, and the approximate characteristic line for the amount of exhaust gas passing through the exhaust valve in each of the first and second half periods is calculated, and the amount of internal EGR is calculated based on each approximate characteristic line. As a result, it is considered that the amount of internal EGR can be calculated in a relatively small number of maps, but similar to the first patent document, too, there has been the problem that it becomes necessary to separately calculate the amount of intake air sucked into a cylinder.
Further, in the conventional technique described in the above-mentioned third patent document, the amount of internal EGR is calculated based on the amount of residual gas at the time when the intake valve is open, and the amount of residual gas which is blown back in the valve overlap period. As a result, it is considered that it is possible to calculate the amount of internal EGR in a relatively small number of maps, but in this case, too, similar to the above-mentioned first patent document, there has been the problem that it becomes necessary to separately calculate the amount of intake air sucked into a cylinder.
However, when the amount of internal EGR, which remains in a cylinder without being discharged from an exhaust valve, and the amount of intake air to be sucked into the cylinder, which comes in from an intake valve, should be normally considered in an integral manner.
For example, it is considered that even with the same internal pressure in the intake pipe, in cases where the amount of internal EGR is large, the amount of intake air sucked into a cylinder becomes smaller, whereas in cases where the amount of internal EGR is small, the amount of intake air sucked into a cylinder becomes larger.
Here, note that in the past, as a calculation device for an amount of intake air in a cylinder, there has been proposed a technique in which in an AFS (air flow sensor) method, an intake system is modeled only from the law of conservation of mass so as to calculate an amount of intake air sucked into a cylinder (for example, refer to a fourth patent document).
In the fourth patent document (refer to paragraphs [0023], [0024], and [0038]-[0042]), the amount of intake air sucked into a cylinder is estimated with a sufficient degree of accuracy by controlling an engine in a suitable manner by means of a simple physical model using a volumetric efficiency corresponding value (a volumetric efficiency correction factor Kv) of air coming into the cylinder from an intake pipe.
It will be considered that if the relation between the volumetric efficiency correction factor Kv and the internal EGR rate shown in the fourth patent document becomes clear, the amount of intake air sucked into a cylinder and the internal EGR rate therein can be calculated with good accuracy at the same time.
However, in any of well-known literatures, there has not been disclosed such a technique which suggests the relation between the volumetric efficiency correction factor Kv and the internal EGR rate.
That is, in cases where a simple physical model such as the one described in the fourth patent document is used as an estimation device for an amount of intake air in a cylinder of an internal combustion engine, there has been a problem that the amount of intake air in the cylinder and the internal EGR rate therein can not be calculated with good accuracy at the same time, due to the fact that the relation between the volumetric efficiency correction factor Kv and the internal EGR rate is not clear.